Poly(hexahydrotriazine) polymers (PHTs) are an emerging class of high strength engineering thermoset polymers that have a unique combination of properties. They have high modulus, solvent resistance, and resistance to environmental stress cracking, yet they can be easily recycled by decomposition to monomers in a strong acid. PHTs may be obtained from heat treatment of a poly(hemiaminal) (PHA) polymer, which may be dissolved in a polar solvent. This enables the preparation of PHT films by solvent casting.
A new subset of PHAs and PHTs may be altered by ultraviolet (UV) irradiation of photoresponsive groups bonded to a PHA or a PHT polymer chain and/or a PHT polymer network. The photoresponsive groups engage in the bonding and/or debonding of portions of the polymer and/or the greater polymer network when exposed to UV light. Some of these photo-induced chemical reactions are referred to as photodimerisation reactions. Reactions of this type may proceed by either a [4π+4π] or [2π+2π] cycloaddition mechanism that can be reversed upon application of an appropriate wavelength of light. In the case of the [2π+2π] cycloaddition reaction, photodimerisation occurs between two alkenes to form a cyclobutane dimer. Some examples of small molecules and photodimerisation reactions are presented in FIG. 1, including the respective wavelengths of light that may be used in the photoreversible reactions.
The new photoresponsive PHAs and PHTs presented herein uniquely provide a rigid high modulus and chemically resistant thermoset polymer and that may be reversibly altered by UV irradiation. Processing of such photoresponsive polymers may be non-destructive and may be performed in localized areas of a polymer film to create patterns in the polymer film.